1. Field of the Invention
The present invention relates to a radio communication apparatus, a radio communication method, a communication control apparatus, and a program.
2. Description of the Related Art
Recently, in accordance with spread of broadband, data of large file size such as video and audio have been transmitted and received on a network. A stream type transmission method is one of transmission methods utilized for transmitting multimedia files such as video and audio. With a download type transmission method in the related art, the file cannot be replayed until the file transfer is completed. However, with the stream type transmission method, received data can be replayed in parallel to the data transfer. Accordingly, the stream type transmission method has been utilized for services such as Internet telephone, video telephone, and video on demand.
An RTP (real-time transport protocol) method defined in IETF RFC3550 is one of Internet technologies appropriate for the stream type transmission method. In data transfer with the RTP method, timestamp is added to a packet as time information. Accordingly, temporal relation between a transmitting side and a receiving side can be perceived, so that synchronized replay can be performed without being affected by delay fluctuation (i.e., jitter) in packet transfer.
Here, the RTP method does not guarantee data transfer in real time. Since priority, setting and managing of packet delivery are not included in transport services provided by the RTP, RTP packets have possibility of causing delivery delay and packet loss similarly to other packets.
A forward error correction encoding method (hereinafter, also called FEC method) utilized at an application layer is a technique to improve reliability of data transfer utilizing the RTP method. The FEC method is a recovery technology of lost packets by utilizing redundant encoding. A communication apparatus at the transmitting side generates a redundant packet (i.e., a parity packet) from a transmission packet and adds to the transmission packet and a communication apparatus at the receiving side decodes the transmission packet by utilizing the added redundant packet, so that a lost packet can be recovered (for example, Japanese Patent Application Laid-Open 2008-160499).
By the way, recently, MIMO (multiple-input multiple-output) utilizing plural transmission antennas has been recognized as an effective method in radio transmission of high bit rate and has been actualized at a physical layer of a radio system such as IEEE802.11n and a MAC (media access control) layer.
SDM (space division multiplexing) and STBC (space-time block coding) are included in typical signal processing modes for communication in the MIMO. With the SDM, space streams are multiplexed by simultaneously transmitting plural space streams from plural transmission antennas, so that the transmission bit rate speeds up (for example, see A. van Zelst, R. van Nee, and G. A. Awater, “Space Division Multiplexing (SDM) for OFDM System” IEEE Proceeding of VTC-Spring 2000, pp. 6-10, 1998). With the STBC, error tolerability of transmission bits is enhanced by simultaneously transmitting a space stream under restraint conditions from transmission antennas being more than the number of the space streams (for example, see D. Agrawal, V. Tarokh, A. Naguib, and N. Seshadri, “Space-Time Coded OFDM for High Data-Rate Wireless Communication over Wideband Channels”, IEEE Proceeding of VTC '98, pp. 2232-2236, 1998).
With the SDM among the above, transmission data is distributed into plural streams and sent out by utilizing plural antennas at the transmitting apparatus side and a signal process utilizing channel characteristics is performed against space multiplexed signals received from plural antennas at the receiving apparatus side, so that space is separated. Accordingly, signals for each stream can be taken out without crosstalk. According to the MIMO communication method, expansion of transmission capacity is achieved in accordance with the number of antennas without widening the frequency band, so that communication speed can be improved. Since the plural space streams are simultaneously transmitted from the plural transmission antennas, the SDM is remarkably effective in transmission rate acceleration. However, as the transmission characteristics being dependent on transmission conditions, there is a disadvantage that the transmission performance is drastically lowered in the case that propagation passage correlation between antennas is high or that SNR (signal to noise ratio) is low, for example.
Meanwhile, with the STBC, transmission information is transmitted with restraint conditions from the plural transmission antennas. That is, transmission is performed as enhancing redundancy. Accordingly, compared to the SDM, speeding up of the transmission rate cannot be achieved. However, there is an advantage that deterioration of transmission performance is small and high diversity effect can be obtained even in the case of high propagation passage correlation between antennas or in the case of low SNR.
Accordingly, in the radio communication apparatus utilizing the MIMO, an apparatus to utilize two modes of the SDM and the STBC as dynamically switching in accordance with radiowave situations has been proposed.